Modulation of angiogenesis through targeting of cysteine oxygenase activity

ABSTRACT

The invention provides methods and compositions for modulating angiogenesis in a subject. The methods of modulating angiogenesis in a subject include administering to the subject a modulator of N-terminal cysteine oxygenase activity. The invention also provides a method of identifying such a modulator and a method of in vitro screening for modulators of N-terminal cysteine oxygenase activity. Additionally, the invention provides a method of treating an angiogenesis-related disorder.

RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Ser. No. 60/366,218, filed Mar. 21, 2002 and U.S. Ser. No. 60/366,207, filed Mar. 21, 2002, the entire contents of which are incorporated herein by reference.

GOVERNMENT INTERESTS

[0002] This invention was made in part with government support under Grant No. GM31530 awarded by the National Institutes of Health. The United States government may have certain rights in this invention.

FIELD OF THE INVENTION

[0003] The invention relates generally to methods of identification of modulators of angiogenesis and more specifically to modulators of N-terminal cysteine oxygenase activity.

BACKGROUND INFORMATION

[0004] Substrates of the ubiquitin (Ub)-dependent N-end rule pathway include proteins with destabilizing N-terminal residues. A set of amino acids that are destabilizing in a given cell yields a rule, called the N-end rule, which relates the in vivo half-life of a protein to the identity of its N-terminal residue. The essential component of a degradation signal called the N-degron is a destabilizing N-terminal residue of a protein. Variants of the N-end rule pathway are present in all organisms examined, from animals and plants to fungi and prokaryotes (FIG. 1A). In eukaryotes, an N-degron of a protein consists of a destabilizing N-terminal residue and an internal lysine, the latter being the site of formation of a substrate-linked multi-Ub chain. The N-end rule has a hierarchic structure (FIG. 1 A). N-terminal Asn and Gln are tertiary destabilizing residues in that they function through their deamidation, by N-terminal amidohydrolases, to yield the secondary destabilizing residues Asp and Glu. The activity of N-terminal Asp and Glu requires their conjugation, by ATE I-encoded Arg-tRNA-protein transferases (R-transferases), to Arg, one of the primary destabilizing residues. The latter are recognized by the E3 (Ub ligase) components of the N-end rule pathway (FIG. 1A). In mammals, the set of destabilizing residues that function through their arginylation contains not only Asp and Glu but also Cys, which is a stabilizing (non-arginylated) residue in the yeast Saccharomyces cerevisiae. The known species of mammalian R-transferase, ATE1-1 and ATE1-2, are produced through alternative splicing of ATE1 pre-mRNA, and have the same substrate specificity as the yeast R-transferase, they arginylate N-terminal Asp or Glu, but cannot arginylate N-terminal Cys.

[0005] Angiogenesis is the growth of new blood vessels. Generally in the body, a balance of angiogenesis growth factors and angiogenesis inhibitors keeps this process of blood vessel growth under control. However, a lack or excess of either angiogenesis growth factors or angiogenesis inhibitors can cause undesired growth of blood vessels or a failure to produce blood vessels.

SUMMARY OF THE INVENTION

[0006] The present invention is based on the seminal discovery that the N-terminal cysteine of a polypeptide is specifically oxidized in vivo, and that it is the oxidized form of N-terminal cysteine, rather than the initial (unoxidized) cysteine moiety, that is actually arginylated by R-transferase.

[0007] In one embodiment, the invention is directed to a method of modulating angiogenesis in a subject. The method of the invention includes administering a modulator of cysteine oxygenase to a subject.

[0008] In another embodiment, the invention provides a method of identifying a modulator of cysteine oxygenase. The method includes providing a cell that expresses a reporter protein with an N-terminal cysteine residue, contacting that cell with at least one potential modulator and measuring the level of the reporter protein expressed in the presence and absence of the modulator. A change of the level of reporter protein expressed in the presence of the modulator as compared to in the absence of the modulator is indicative of modulation. Such a change may be an increase in expression or a decrease in expression. The level of reporter protein may be indicative of the level of protein expressed or may be indicative of the half life of the reporter.

[0009] In another embodiment, the method further includes providing a second cell that expresses a second reporter protein with an N-terminal argenine, aspartic acid or glutamic acid residue. The second cell is contacted with at least one potential modulator of cysteine oxygenase and the level of second reporter protein expressed within the second cell relative to the level of reporter protein expressed within the first cell is measured.

[0010] In still another embodiment, the invention provides an in vitro method of screening for modulators of cysteine oxygenase including contacting one or more potential modulators of cysteine oxygenase with a cell extract, providing a peptide substrate having an N-terminal cysteine residue and determining the level of N-terminal cysteine oxidation of the peptide substrate. A change of the level of N-terminal cysteine oxidation in the presence of the modulator as compared to in the absence of the modulator is indicative of modulation. Such a change may be an increase in expression or a decrease in level of oxidation.

[0011] In yet another embodiment the invention is directed to a method of treating an angiogenesis-related disorder in a subject. The method includes administering a modulator of angiogenesis to a subject.

[0012] The invention also includes a modulator identified by a method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1. N-terminal residues are indicated by single-letter abbreviations for amino acids. Ovals denote the rest of a protein substrate. Type 1 and type 2 primary destabilizing N-terminal residues (Arg, Lys, His, Phe, Leu, Trp, Tyr, and Ile) are recognized by functionally overlapping Ub ligases (E3s) that include UBR1 (E3α) and UBR2. N-terminal Ala, Ser and Thr are recognized by a distinct, unidentified E3.

[0014]FIG. 2A shows quantitation of results of the stabilization of N-end rule substrates in mouse ATE1^(−/−) cells, and oxidation-arginylation of N-terminal cysteine. ATE1^(+/+) and ATE1^(−/−) EF cells were transfected with plasmids expressing ^(f)DHFR^(h)-Ub^(R48)-X-nsP4^(f), whose cleavage yielded the ^(f)DFHR^(h)-Ub^(R48) reference protein, denoted as DHFR, and X-nsP4^(f) test proteins (X=Met, Arg, Asp, Glu, Cys), denoted as X-nsP4. Cells were pulse-labeled for 10 min with ³⁵S-methionine, and chased for 1 and 2 hr. For each time point, the ratio of ³⁵S in X-nsP4^(f) (X=Arg, Asp, Glu, Cys) to ³⁵S in the ^(f)DHFR-Ub^(R48) reference protein at the same time point, was plotted as the percentage of this ratio relative to that for Met-nsP4^(f)(which bore a stabilizing N-terminal residue) at time 0. Open and closed symbols: +/+ and ATE1^(−/−) cells, respectively. Δ, ▴, Met-nsP4^(f); ∘, , Arg-nsP4^(f); □, ▪, Asp-nsP4^(f); ∇, ▾, Glu-nsP4^(f); ⋄, ♦, Cys-nsP4^(f).

[0015]FIG. 2B shows a cell-free assay for mouse R-transferase using ³H-Arg, S105 extracts from +/+and ATE1^(−/−) EF cells, and unlabeled X-βgals (X=Cys, Asp, Glu, Met, Arg) and α-lactalbumin as test substrates. Arginylated test substrates are indicated on the right. Asterisks indicate arginylated endogenous proteins in the extracts.

[0016]FIGS. 2C and 2D show Mouse ATE1^(−/−) EFs were co-transfected with a plasmid expressing X-nsP4^(f) and a plasmid expressing either wild-type S. cerevisiae ATE1 (pCDNA3yATE1) or its enzymatically impaired mutant (pCDNA3yATE1C23A). Curve designations: Met-nsP4^(f) coexpressed with either a vector alone (open gray square), or yeast ATE1 (solid gray square), or mutant yeast ATE1 (solid black square). Cys-nsP4^(f) the same except for diamonds instead of squares. Asp-nsP4^(f): the same except for circles instead of squares.

[0017]FIG. 2E shows the N-terminal sequences, as determined through Edman degradation, of mouse RGS4-flag-His₆ isolated from mouse L cells. Calk denotes alkylated Cys residue.

[0018]FIG. 2F shows the N-terminal sequence of mouse RGS4 deduced from mass spectrometric data in shown in FIG. 2G. The M_(r) of residue 2 (Cys-2 in the arginylated RGS4) identified it as cysteic acid (M_(r) of 151.1 Da, 48.1 Da higher than M_(r) of Cys). M* denotes homoserine lactone.

[0019]FIG. 2G shows a mass spectrometric analysis of the CNBr-produced 19-residue N-terminal peptide of RGS4. Both the observed molecular masses of peaks and the differences between observed and expected values are shown.

[0020]FIG. 2H shows the chemical formulas of N-terminal Cys residue, Cys sulfinic acid, cysteic acid, and Asp residues.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Methods are provided herein, based on the discovery that the N-terminal cysteine of a polypeptide is specifically oxidized in vivo, and that it is the oxidized form of N-terminal cysteine that is arginylated by R-transferase. In particular, the invention provides methods for modulating angiogenisis in a subject by providing a therapeutically effective amount of a modulator of enzymatic oxidation of N-terminal cysteine in polypeptides. The invention described herein provides screening methods for modulators of cysteine oxygenase, and methods for treating an animal by providing a modulator of N-terminal cysteine oxidation.

[0022] As used herein, the term “modulate” or “modulation” is used with regard to modulation of activity. In particular, the present invention provides modulators of N-terminal cysteine oxidation. As used herein, modulate refers to changing the normal activity. Modulation may be inhibition of an activity or stimulation of the same.

[0023] As used herein, the term “angiogenesis” is used to refer to the growth of new blood vessels. Accordingly, an “angiogenesis-related disorder” is a disorder in which the activity of angiogenesis is altered. Such diseases may be completely caused by angiogenesis or may be exacerbated by angiogenesis activity. Angiogenesis-related disorders due to excessive angiogenesis may include, but are not limited to, cancer, tumors, rheumatoid arthritis, psoriasis, rosacea and metastasis of cancerous cells. Angiogenesis-related disorders due to insufficient angiogenesis may include, but are not limited to, coronary artery disease, stroke, ulcers and delayed wound healing.

[0024] In one embodiment, the invention provides methods for screening for modulators of cysteine oxygenase, in which one or more potential modulators of cysteine oxygenase are contacted with a first cell. The cell expresses a first reporter protein having an N-terminal cysteine residue. The level of said first reporter protein present is then determined in the cell either directly or through an indirect means. A change of the level of reporter protein expressed in the presence of the modulator as compared to in the absence of the modulator is indicative of modulation. Such a change may be an increase in expression or a decrease in expression. The level of reporter protein may be indicative of the level of protein expressed or may be indicative of the half life of the reporter. In another embodiment of the invention, the expression of the reporter protein is inducible.

[0025] The reporter protein is selected such that it has a relatively short half-life in the cell. Generally, the reporter protein has a half-life in the first cell of less than about an hour in the absence of a modulator. The reporter protein can have a half-life in the first cell of less than about 10 minutes in the absence of the modulator.

[0026] In one embodiment of the invention, the reporter protein is expressed as a cleavable fusion protein. The cleavable fusion protein can have an ubiquitin domain functionally linked to the reporter protein. In an appropriate cell, the ubiquitin domain is rapidly cleaved by one of several deubiquitylating enzymes.

[0027] In yet another embodiment of the invention, the screening methods described can be accomplished in a variety of cells, including in a cultured mammalian cell, a yeast cell or a bacterial cell.

[0028] The first type of screen for inhibitors of ATE1-mediated N-terminal arginylation can be carried in the setting of either mammalian cells in culture or yeast (S. cerevisiae) cells in culture. In mammalian cells and yeast, the ATE1-encoded R-transferases are solely responsible for N-terminal arginylation. The logic of either yeast- or mammalian cells-based screens is essentially the same. The advantage of a yeast-based screen is the ease of handling and analyzing yeast cell cultures; the disadvantage is that potential inhibitors of R-transferase that would cross the plasma membrane of mammalian cells may be incapable of gaining entry into yeast cells, given substantial differences in the permeability (and transport) properties of the plasma membrane between yeast and mammals. A mammalian cells-based screen is preferred, since the sought inhibitors would be intended for use in mammals. The screen: a mammalian (mouse or human) cell line is set up that expresses a short-lived reporter whose ubiquitin-dependent degradation by the N-end rule pathway involves N-terminal arginylation. Such a reporter would be a protein with either a genetically selectable or “screenable” (visually detectable) influence on the test cell culture.

[0029] A number of classes of reporter proteins are suitable for use with the methods of the invention. For example, the reporter protein can be a genetically selectable marker protein, and the relative level of the reporter protein in the presence and absence of a modulator can accomplished indirectly through a selective genetic screen which requires the presence of said reporter protein for survival of said first cell. In such an example, a modulator which inhibits the cysteine oxygenase activity would therefore lengthen the half-life of the reporter protein. For example, the selectable marker protein can be an antibiotic resistance protein. In one such example, the reporter protein is designed to be short-lived in a cell, the cell will be relatively sensitive to the relevant antibiotic. By contrast, if the protein is made long-lived in vivo, for example through the inhibition of a proteolytic pathway, and more specifically through inhibition of cysteine oxygenase activity, that normally destroys this protein, its steady-state level would, by definition, increase, and the cell would become resistant to the same dose of antibiotic. This readout would enable a selection-based screen. A number of reporter proteins that confer antibiotic resistance are known to those skilled in the art.

[0030] Detection of the relative abundance of the reporter protein also can be accomplished by more direct means. For example, the reporter protein can be a screenable reporter protein. Such proteins may include, but are not limited to Green Fluorescent Protein (GFP) and E. coli β-galactosidase (βgal). A number of fluorescent proteins are known to those of skill in the art. These proteins, if expressed in a cell, can be detected either through their fluorescence (GFP) or through their enzymatic activity (βgal). The suitable reporter protein can be expressed in the cell type used for the assay and has sufficient fluorescence intensity to be detected within the cell at the appropriate concentrations. Making a reporter of this class short-lived in vivo would strongly diminish its steady-state level in a cell. If degradation of such a short-lived reporter is inhibited, its steady-state level will rise, enabling the detection of reporter. The level of protein present can be determined using fluorescence detection, for example, using a fluorometer or fluorescence microscope. The relative amount of fluorescence can be compared between cells treated with a potential modulator versus no treatment. Similarly, the reporter protein can be a light-generating protein, for example, luciferase. The amount of light produced can be measured and compared in cells treated with a modulator and in untreated cells.

[0031] The level of reporter protein present can also be determined enzymatically. For example, enzymes capable of making a colorimetric change in a substrate, can be detected indirectly. One such protein, E. coli β-galactosidase (βgal), can be detected through a colorimetric change it causes in the substrate β-gal (5-bromo-4-chloro-3-indolyl-β-D-galactosidase). Making a reporter of this class short-lived in vivo would strongly diminish its steady-state level in a cell. If degradation of such a short-lived reporter is inhibited, its steady-state level will rise, enabling the detection of the reporter protein (through an enzymatic reaction).

[0032] Other reporter protein systems are known to one of skill in the art and can be applied to the methods of the invention. As will be apparent to one of skill in the art, the invention is not limited to those currently known, but can also be extended to newly discovered reporter protein systems.

[0033] The invention also provides modulators of N-terminal cysteine oxygenase activity. In summary, the invention provides methods for identifying modulators of cysteine oxygenase that modifies the N-terminal cysteine of a polypeptide. This modulation is also essential for a cysteine-specific subset of N-terminal arginylation reactions, which, in turn, are important for cardiovascular functions, including angiogenesis.

[0034] According to another embodiment of the invention, the screening methods can include additional features, including contacting the potential modulators of cysteine oxygenase with a second cell, wherein the second cell expresses a second reporter protein having an N-terminal arginine, aspartic acid or glutamic acid residue and determining the level of expression of said second reporter protein relative to said first reporter protein. These settings serve as negative-control settings for those in which a reporter protein contains N-terminal cysteine. According to the invention, the second cell can be the same cell as the first cell (i.e., a cell expressing both the first reporter protein and the second reporter protein) or can be a different cell (i.e., one not expressing the first reporter protein). Such a screening method allows for the selection of modulators specific for the cysteine oxygenase activity that do not inhibit the rest of the N-end rule pathway, including the rest of its arginylation branch. According to this embodiment of the invention, a desired modulator of cysteine oxygenase activity would alter the relative abundance of the N-terminal cysteine-containing first reporter protein, but would not significantly alter the relative abundance of the second reporter protein having an arginine, aspartic acid or glutamic acid residue at the N-terminus. In one embodiment of such an assay system, the first reporter protein and the second reporter proteins are fluorescent proteins having distinct spectral properties.

[0035] In one embodiment of the invention, the screen is carried out with a GFP-based reporter. Mammalian cells growing in microtiter plates and expressing Cys-GFP that is designed to be N-terminally arginylated and rapidly destroyed by the N-end rule pathway, are exposed to a library of potential inhibitors of cysteine oxidase (one tested compound per well, with different concentrations of a compound in adjacent wells). After incubation with the library of compounds, the microtiter plates are scanned by plate fluorimeter to detect wells where the levels of Cys-GFP increased. Compounds of the library that caused the increase of GFP level are then re-tested in control assays, to identify those among them that act by inhibiting oxidation of the N-terminal cysteine or its subsequent arginylation. One way to do that is to add these compounds, one at a time, to an otherwise identical mammalian cell culture that expresses Asp-GFP, a reporter identical to Cys-GFP except having Asp instead of Cys at its N-terminus. Such a reporter would be degraded by the N-end rule pathway in arginylation-dependent manner, but would not require the action of the cysteine oxygenase activity. Likewise, the screen can differentiate between compounds that act on the cysteine oxygenase versus the rest of the N-end rule pathway, including the rest of its arginylation branch. For example, these compounds can be added, one at a time, to an otherwise identical mammalian cell culture that expresses Arg-GFP, a reporter identical to Cys-GFP and Asp-GFP except having Arg residue at its N-terminus. Such a reporter would be degraded by the N-end rule pathway in arginylation-independent manner; and therefore it would not require the action of the cysteine oxygenase activity or the arginylation activity. Any compound that inhibits the enzymatic oxidation of N-terminal cysteine would be, immediately, a strong candidate for an inhibitor of N-terminal cysteine oxidation. Any compound that inhibits degradation of both Cys-GFP and Asp-GFP but does not significantly inhibit degradation of Arg-GFP would be, immediately, a strong candidate for a specific inhibitor of R-transferase.

[0036] Confirmation of inhibition of R-transferase may then be obtained in at least two ways: Whether this is, in fact, the case could then be determined straightforwardly in at least two ways: 1) in vivo, by isolating Asp-GFP from cells treated with inhibitor versus control (untreated) cells, and N-terminally sequencing Asp-GFP from these two sources, to determine whether its N-terminal arginylation was, in fact, inhibited in vivo by the compound in question; or 2) a direct enzymatic assay with purified R-transferase and test the identified compound or compounds (identified through initial screen) for inhibition of N-terminal arginylation of a model substrate in vitro.

[0037] To carry out a screen for inhibitors of N-terminal cysteine oxidation through linkage to arginylation and degradation, a reporter of either class will have to be extended N-terminally with a sequence of amino acid residues known as the N-degron, a class of degradation signals (degrons) recognized by the universally present, ubiquitin-dependent N-end rule pathway of protein degradation. A particular class of N-degrons that would require N-terminal arginylation for its activity would have cysteine (Cys) as the N-terminal residue of the reporter, since oxidized Cys is arginylated by the N-end rule pathway, by R-transferase, prior to reporter's degradation. Previous extensive work by this and other laboratories made the construction of N-degron-equipped reporters a well-developed art known to researchers in the field.

[0038] A number of techniques are available to express a reporter protein having a desired N-terminal residue. Universally, translated proteins are initiated with an amino-terminal methionine residue. Processing of the protein by various peptidases can expose an internal residue as an amino-terminal residue. One such technique involves expressing a fusion protein having a ubiquitin domain and a reporter protein domain. The ubiquitin domain is rapidly cleaved from the fusion protein by a class of enzymes known as the deubiquitinylating enzymes (DUBs). This strategy is well known to one of skill in the art and is taught, for example, in references 3, 5 and 16, which are incorporated herein by reference in their entireties.

[0039] The invention also provides in vitro methods for screening for modulators of cysteine oxygenase. The method involves contacting one or more potential modulators of cysteine oxygenase with a cell extract, providing a peptide substrate having an N-terminal cysteine residue; and determining the level of cysteine oxidation of said peptide substrate. A change of the level of cysteine oxidation in the presence of the modulator as compared to in the absence of the modulator is indicative of modulation. Such a change may be an increase in oxidation or a decrease in oxidation.

[0040] The level of cysteine oxidation in the peptide substrate can be determined by any suitable method including, for example, gel electrophoresis, capillary electrophoresis, thin layer chromatography (TLC); high pressure liquid chromatography (HPLC), mass spectrometry and immunoassay. The type of detection used in such a scheme can be determined by one of skill in the art and will be chosen as to be compatible with the type of separation used. For example, when the technique to be used is HPLC, detection can be UV detection. The degree or rate of N-terminal cysteine oxidation can be compared using such techniques, or combinations of techniques to determine the impact of a modulator of cysteine oxygenase activity.

[0041] The cysteine oxidation level also can be determined using an antibody based system. An antibody is generated that selectively recognizes the peptide substrate having an N-terminal cysteine residue versus a peptide having an oxidized N-terminal cysteine residue. In such an arrangement, a peptide substrate having an N-terminal cysteine residue is covalently or otherwise affixed to the surface of a vessel, for example the well of a microtiter plate. A cell extract having N-terminal cysteine oxygenase activity is added to the wells of such a plate in the presence or absence of a potential modulator of the cysteine oxidase activity. The degree of cysteine oxidation is determined using the antibody. A number of suitable detection schemes are known to one of skill in the art of immunological assays, including directly linking the antibody to a colorimetric or fluorescent moiety, or using a sandwich type assay, in which the detection means is provided by a second protein, typically an antibody which binds to the first antibody.

[0042] In still another embodiment, the invention provides modulators of cysteine oxygenase identified by any of the methods of the invention. In yet another embodiment, the modulators of cysteine oxygenase are also modulators of angiogenesis.

[0043] In another embodiment the invention provides methods for modulating angiogenesis in a subject, including administering to a mammal a modulator of cysteine oxygenase. In one embodiment, the subject is a mammal, such as a human or a mouse. Inhibitors of cysteine oxygenase activity can be used to treat patients, for example, who are in need of inhibited angiogenic activity, for example, those with tumors.

[0044] In still another embodiment of the invention, a method of treating an angiogenesis-related disorder is provided. The method involves administering a modulator of angiogenesis to a subject, thereby treating the angiogenesis-related disorder in the subject. Such a modulator may inhibit cysteine oxygenase. The subject may be a mammal. The angiogenesis modulator may inhibit or stimulate angiogenesis in the subject, thereby treating the angiogenesis-related disorder. Angiogenesis-related disorders treated by this method of the invention may include, but are not limited to: cancer, tumors, rheumatoid arthritis, psoriasis, rosacea, metastasis of cancerous cells, coronary artery disease, stroke, ulcers and delayed wound healing.

[0045] In one embodiment, the potential modulators are small organic compounds, compounds having a molecular weight of less than about 1,000. Such compounds are available from natural sources including plant, bacterial, and fungal extracts, from techniques such as combinatorial chemistry. One of skill in the art will recognize that libraries of compounds are commercially available.

[0046] It is noted that Cys dioxygenase (CDO) converts free Cys, in the presence of oxygen, to Cys sulfinic acid (FIG. 1). A rigorous identification of the physiologically relevant enzyme(s) that oxidizes N-terminal Cys will require genetic analysis of a candidate enzyme(s).

[0047] The following examples are intended to illustrate but not limit the invention.

EXAMPLE 1 Assays of Arginyl Transferase Activity

[0048] Mouse ATE1 was isolated using screening, with an ATE1 cDNA fragment (nt 638-1,491), a BAC library (Genome Systems) from 129/SvJ ES mouse cells. The exon/intron organization of the first˜20 kb of ATE1 was determined using exon-specific PCR primers to produce genomic DNA fragments flanked by exons. The targeting vector was linearized with HindIII and electroporated into CJ7 embryonic stem (ES) cells, followed by selection and identification of the correctly targeted ATE1^(+/−) ES cell clones with normal karyotype. Standard techniques were then used to produce chimeric and ATE1^(−/−) mice. Phenotypes of ATE1^(−/−) embryos were observed mainly with mice of the C57BL/6J-129SvEv (mixed) background, and confirmed in the 129SvEv (inbred) background. RT-PCR, Southern and Northern analyses, and PCR-mediated genotyping of embryos and pups were performed. Standard procedures were used for thin sectioning and staining with hematoxylin/eosin, X-Gal, or anti-PECAM-1 antibody (clone MEC 13.3, Pharmingen). The TUNEL assay was performed using a kit (Roche) and fluorescein-dUTP. For R-transferase assays (FIG. 2B), Ub-X-bgal proteins were purified from E. coli carrying pKKUbXbgal plasmids. The R-transferase reaction (50 ml) contained S105 supernatant (0.5 mg of protein per ml) from either EF cells or whole embryos, Ub-X-βgal or α-lactalbumin (0.2 mg/ml), E. coli tRNA (1 mg/ml), E. coli aminoacyl-tRNA synthetases (50 mg/ml), puromycin (0.2 mM), bestatin (0.15 mM), 5 mM MG132 (proteasome inhibitor), 0.4 mM Lys-Ala dipeptide (inhibitor of post-arginylation steps in the N-end rule pathway), 1 mM ATP, 10 mM creatine phosphate, 0.1 M KC1, 5 mM MgCl₂, 50 mM b-mercaptoethanol, 50 mM Tris-HC1 (pH 8.0) and 0.3 mM ³H-arginine (New England Nuclear). The reaction mixture was incubated for 3 hr (2 hr with embryo extracts) at 37° C. A 20 ml sample was precipitated with 10% TCA, and analyzed by SDS-12% PAGE and fluorography.

[0049] Primary mouse EFs were established from E13.5 ATE1^(−/−) and littermate +/+ embryos and immortalized to increase transfection efficiency. Cells were transiently transfected with pcDNA3flagDHFRhaUbXnsP4flag, which expressed ^(f)DFHR^(h)-UbR48-X-nsP4^(f) (and the main text) from the PCMV promoter. Cells were labeled with 35S-EXPRESS (New England Nuclear) for 10 min at 37° C., followed by a chase for 0, 1, and 2 hr in the presence of cycloheximide, preparation of extracts, precipitation with anti-flag antibody, SDS-10% PAGE, autoradiography, and quantitation using PhosphorImager. In other pulse-chases, ATE1^(−/−) EFs were co-transfected with a plasmid expressing X-nsP4^(f)(^(f)DFHR^(h)-UbR48-X-nsP4f) and either pCDNA3yATE1, expressing S. cerevisiae ATE1, or pCDNA3yATE1C23A (derived from a plasmid supplied by Dr. C. Pickart, Johns Hopkins University), which expressed ATE1C23SA, bearing Cys→Ala mutation at position 23.

EXAMPLE 2 Arginylation of N-terminal Cysteine-Containing Polypeptides

[0050] To measure the N-terminal arginylation directly, either purified Ub-X-βgal proteins (X=Met, Arg, Glu, Cys) or purified human α-lactalbumin (bearing N-terminal Glu) were added to +/+ and ATE1^(−/−) immortalized embryonic fibroblast (EF) cell extracts supplemented with ATP, total E. coli tRNA and a mixture of E. coli aminoacyl-tRNA synthetases. SDS-PAGE and fluorography were used to detect covalent conjugation of ³H-Arg to test proteins in these extracts (FIG. 2B). Ub-X-βgals are rapidly deubiquitylated in vivo and in cell-free extracts, yielding X-βgal test proteins. As expected, Asp-βgal, Glu-βgal and α-lactalbumin were arginylated in the extracts from +/+ EF cells, whereas Arg-βgal and Met-βgal, bearing a primary destabilizing and a stabilizing N-terminal residue, respectively, were not arginylated (FIG. 2B). Crucially, no arginylation of Asp-βgal, Glu-βgal and α-lactalbumin could be detected in ATE1^(−/−) EF extracts (FIG. 2B), even after prolonged fluorographic exposures. Identical results were obtained with extracts from +/+ and ATE1^(−/−) embryos. In addition to being consistent with the conclusions from pulse-chase analyses in EF cells (FIG. 2A), these findings confirmed the absence of R-transferase activity from ATE1^(−/−) embryos.

[0051] The N-terminal Cys, of Cys-βgal, was not arginylated in either +/+ or ATE1^(−/−) extracts, in contrast to N-terminal Asp and Glu (FIG. 2B), suggesting that the previously observed arginylation of N-terminal Cys, and the demonstrated ATE1 dependence of the in vivo degradation of Cys-bearing N-end rule substrates in mouse cells (FIG. 2A) involved a modification of N-terminal Cys prior to its arginylation. In this interpretation, the absence of arginylation of Cys-βgal in an extract from +/+ mouse cells could be caused, for example, by inactivation of a Cys-modifying enzyme in the extract. A comparison of Asp and Cys structures suggested that either the Cys sulfinic acid residue (CysO₂, an oxidized derivative of Cys) or the cysteic acid residue (CysO₃, a further oxidized Cys derivative), may be sufficiently close in structure and charge distribution to Asp (FIG. 2H) to serve as a substrate of R-transferases. Consistent with this possibility, a protease called Asp-N has been shown to cleave peptide bonds N-terminal to either the Asp or CysO₃ residues. Another class of enzymes, aspartate aminotransferases, can utilize either Asp or oxidized Cys as substrates.

[0052] This example illustrates that degradation of a test protein bearing N-terminal Cys is tRNA-dependent (implying the involvement of R-transferase), similarly to the degradation of otherwise identical proteins bearing N-terminal Asp, Glu, Asn or Gln, and in contrast to degradation of otherwise identical proteins bearing primary destabilizing N-terminal residues (FIG. 1). The normally short-lived Cys-bearing N-end rule substrates are stabilized in mouse ATE1^(−/−) cells (FIG. 2A), although the ATE1-encoded R-transferases that are absent from these cells cannot arginylate N-terminal Cys upon their expression in atelΔS. cerevisiae.

EXAMPLE 3 Generation of RGS4

[0053] To produce RGS4, mouse L cells were transiently transfected with pCDNA3RGS4flagHis₆, which expressed mouse RGS4-flag-His₆ from the PCMV promoter and was constructed from the pcDNA3RGS4 plasmid. Cell extracts were prepared 30 hr later; RGS4-flag-His₆ was purified using Ni-NTA Magnetic Agarose Beads (Qiagen), then treated with 25 mM iodoacetamide in 7 M urea, followed by SDS-PAGE, the transfer onto Immobilon-P membrane, and sequencing by Edman-degradation, using the 476A sequencer (Perkin-Elmer). For mass spectrometry, RGS4-flag-His₆ was treated with 90 mM iodoacetic acid in 8 M urea for 50 min at ˜20° C., then cleaved with CNBr in 55% HCOOH under argon atmosphere for 12 hr in the dark, followed by reverse phase HPLC and on-line, fragmentation-based mass spectrometric sequencing of peptides.

EXAMPLE 4 Arginylation of N-terminal Cysteine-Containing RGS4

[0054] The existence of amino-terminal arginylated Cys residues as CysO₂ or CysO₃ was verified and confirmed with mouse RGS4, a GTPase-activating (GAP) protein that bears N-terminal Cys and was previously shown to be arginylated and degraded by the N-end rule pathway in rabbit reticulocyte extracts. RGS4-His₆ was transiently expressed in mouse L cells, purified, treated with iodoacetamide to alkylate Cys residues (thereby making them identifiable by the sequencing procedure used), and was N-terminally sequenced by Edman degradation. The results indicated the presence of two RGS4 proteins, an arginylated and unarginylated one, the former being a major species (FIG. 2E). Remarkably, whereas the expected Cys residue at position 12 of arginylated RGS4 could be identified as alkylated Cys, the expected (alkylated) Cys residue at position 2 (position 1 in the unarginylated RGS4) could not be identified by the Edman procedure (FIG. 2E), indicating that a residue at this position existed as a derivative of Cys prior to alkylation, and thereby precluded it. To determine the identity of a residue at position 2, the purified, alkylated RGS4 was cleaved with cyanogen bromide (CNBr), followed by HPLC fractionation and on-line mass spectrometric sequencing of CNBr-produced peptides. Mass spectra derived from the arginylated N-terminal peptide of RGS4 demonstrated that the mass of a residue at position 2 was increased by 48 (+0.1) Da in comparison to the expected mass of Cys-2 (FIGS. 2F, G). These results (FIGS. 2E-G) identified residue 2 as a cysteic acid (CysO₃) residue (FIG. 2H).

EXAMPLE 5 Rescue of the Destabilizing Activity of CYS in Mouse ATE1^(−/−) CELLS

[0055] Verification and confirmation of the fact that the yeast R-transferase should be able to rescue the destabilizing activity of Cys in mouse ATE1^(−/−) cells, owing to the presence of Cys-oxidation activity in these cells was obtained by the present example. Pulse-chase assays were carried out with mouse ATE1^(−/−) EF cells that expressed X-nsP4^(f) proteins (X=Met, Asp, Cys) and either the wild-type S. cerevisiae ATE1 (R-transferase) or ATE1^(C23SA), an enzymatically impaired missense mutant. The metabolic stability of long-lived Met-nsP4^(f) (bearing a stabilizing N-terminal residue) was unchanged in the presence of yeast ATE1. In contrast, both Asp-nsP4^(f) and Cys-nsP4^(f), which were long-lived in mouse ATE1^(−/−) EF cells, became short-lived in the presence of yeast ATE1 (FIGS. 2C, D). The complementation by yeast R-transferase required its enzymatic activity, since ATE1^(C23SA), a catalytically impaired missense mutant, had a significantly weaker effect (FIGS. 2C, D). In addition to supporting the Cys-oxidation/arginylation hypothesis, these results suggested that the oxidation of N-terminal Cys is an enzymatic (rather than uncatalyzed) reaction, since the intracellular solvent conditions, including redox potential, are likely to be similar in mammalian and yeast cells. The fact of stoichiometric oxidation of N-terminal Cys in mouse cells (FIGS. 2E-G) indicated the same conclusion.

[0056] Thus, S. cerevisiae R-transferase, which cannot arginylate N-terminal Cys, can rescue the in vivo arginylation and degradation of Cys-bearing N-end rule substrates in mouse ATE1^(−/−) cells (FIGS. 2C, D). Such a cell-based system can be used with the methods of the invention described above to identify modulators of cysteine oxygenase activity, which is necessary for degradation of proteins having an N-terminal cysteine residue.

[0057] In contrast to most of the destabilizing residues, including Asp and Glu (FIG. 1), Cys can be exposed at the N-terminus of a protein substrate by Met-aminopeptidases, which cleave off the N-terminal Met of a newly formed protein if the side chain of a second residue is small enough; only Cys, Ala, Thr, and Ser of the mammalian N-end rule satisfy this condition. Two Cys-bearing mouse proteins, RGS4 and RGS16, were recently identified as N-end rule substrates. A mammalian genome encodes a few hundred proteins containing the N-terminal Met-Cys sequence. However, given the constraints of N-degron's organization, the presence of Cys at the N-terminus of a protein is not, by itself, sufficient to render this protein an N-end rule substrate. In addition, the oxidation (and subsequent arginylation) of N-terminal Cys may compete with its other known modifications, including acetylation and palmitoylation. N-end rule substrates that bear the arginylation-dependent destabilizing N-terminal residues (Asn, Gln, Asp, Glu, and Cys) (FIG. 1) can also be produced through cleavages anywhere in a protein's polypeptide chain. For example, the conditional cleavage of a subunit of the mammalian cohesin complex at the metaphase-anaphase transition is predicted to produce a putative N-end rule substrate whose degradation would require N-terminal arginylation. Since the failure to degrade, through the N-end rule pathway, a cohesin fragment has been shown to impair the fidelity of chromosome segregation in S. cerevisiae, mouse ATE1^(−/−) cells may exhibit an increased chromosome instability.

[0058] The N-end rule is implicated in a variety of physiological processes, including angiogenesis. Mice lacking the gene ATE1 were shown to have extensive defects in angiogenic development. Because of the importance of the arginylation pathway to angiogenesis, inhibitors of the cysteine oxygenase activity can be used as anti-angiogenic compounds for the treatment of tumors, and particularly of solid tumors.

[0059] Recent studies have identified the mammalian Met-aminopeptidase MetAP2 as the target of fumagillin and related inhibitors of angiogenesis. Upon inhibition of MetAP2, some intracellular proteins partially retain their N-terminal Met residues. The N-terminal Met-Cys bond (but neither Met-Asp nor Met-Glu) can be cleaved by MetAP2. The results of the invention suggest that metabolic stabilization of an arginylation-dependent N-end rule substrate(s) in mouse ATE1^(−/−) cells causes angiogenic and cardiogenic defects. Thus, one possibility is that fumagillin and related drugs may act by partially inhibiting the N-terminal Met-Cys cleavage and thereby partially stabilizing an otherwise short-lived repressor(s) of angiogenesis that bears a Cys-containing N-degron. If the repressor's half-life is normally short enough, even a small fraction of repressor molecules that retain Met-Cys and are, therefore, long-lived, would yield a strong increase in the repressor's steady-state level, thereby possibly accounting for the finding that even a partial inhibition of MetAP2 is sufficient to block proliferation of endothelial cells. Likewise, inhibition of the cysteine oxidase pathway also is likely to block such proliferation.

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[0092] Although the invention has been described with reference to the above examples, it will be understood that modifications and variations are encompassed within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims. 

What is claimed is:
 1. A method of modulating angiogenesis in a subject, comprising administering to a subject a modulator of cysteine oxygenase, thereby modulating angiogenesis in the subject.
 2. The method of claim 1, wherein the subject is a mammal.
 3. A method of identifying a modulator of cysteine oxygenase, comprising: a) providing a cell that expresses a reporter protein with an N-terminal cysteine residue; b) contacting at least one potential modulator with the cell; and c) measuring the level of the reporter protein expressed in the presence and absence of the modulator, wherein a change in the level of reporter protein expressed is indicative of modulation of cysteine oxygenase activity.
 4. The method of claim 3, wherein the cell is a cultured mammalian cell.
 5. The method of claim 3, wherein the cell is a yeast cell.
 6. The method of claim 3, wherein the cell is a bacterial cell.
 7. The method of claim 3, wherein the reporter protein is expressed as a cleavable fusion protein comprising a reporter portion and a ubiquitin domain functionally linked to the reporter protein, wherein the reporter portion comprises an N-terminal cysteine.
 8. The method of claim 3, wherein the reporter protein comprises a selectable marker protein, and the measuring comprises a selective genetic screen.
 9. The method of claim 8, wherein the selectable marker protein is an antibiotic resistance protein.
 10. The method of claim 3, wherein the measuring comprises fluorescence detection of the reporter protein.
 11. The method of claim 3, wherein the reporter portion comprises a fluorescent protein.
 12. The method of claim 11, wherein the fluorescent protein is green fluorescent protein.
 13. The method of claim 3, wherein the reporter portion comprises a light-generating protein.
 14. The method of claim 12, wherein the light-generating protein is luciferase.
 15. The method of claim 3, wherein the reporter portion comprises an enzyme.
 16. The method of claim 15, wherein the enzyme is beta-galactosidase.
 17. The method of claim 3, wherein the expression of the reporter protein is inducible.
 18. The method of claim 3, wherein the reporter protein has a half-life in the first cell of less than about an hour in the absence of a modulator.
 19. The method of claim 3, wherein the reporter protein has a half-life in the first cell of less than about 10 minutes in the absence of a modulator.
 20. The method of claim 3, wherein the change in the level of reporter protein expressed is increased expression.
 21. The method of claim 3, further comprising: d) providing a second cell that expresses a second reporter protein, wherein the second reporter portion comprises an N-terminal argenine, aspartic acid or glutamic acid; e) contacting at least one potential modulator of cysteine oxygenase activity with the second cell; and f) measuring the level of second reporter protein expressed within the second cell relative to the level of reporter protein expressed within the first cell.
 22. The method of claim 21, wherein the cell in step a) and the second cell are the same cell.
 23. The method of claim 22, wherein the reporter protein of step a) and the second reporter protein are fluorescent proteins having distinct spectral properties.
 24. A modulator of cysteine oxygenase identified by the method of claim
 3. 25. The modulator of claim 23, wherein the modulator of cysteine oxygenase also modulates angiogenesis.
 26. A method of modulating angiogenesis in a subject, comprising administration of a modulator identified by the method of claim
 3. 27. An in vitro method of screening for modulators of cysteine oxygenase, comprising: a) contacting one or more potential modulators of cysteine oxygenase with a cell extract; b) providing a peptide substrate having an N-terminal cysteine residue; and c) determining the level of N-terminal cysteine oxidation of the peptide substrate, wherein a change in the level of cysteine oxidation of the peptide substrate is indicative of modulation of cysteine oxygenase activity.
 28. The method of claim 27, wherein the determining of the level of cysteine oxidation comprises electrophoresis, chromatography, mass spectrometry or imunoassay.
 29. The method of claim 27, wherein the determining of the level of cysteine oxidation comprises reaction with an antibody that distinguishes between the substrate peptide and an oxidized substrate peptide.
 30. The method of claim 27, wherein the modulator of cysteine oxygenase also modulates angiogenesis.
 31. The method of claim 27, wherein the change in the level of oxidation of the peptide substrate is increased oxidation.
 32. A modulator of cysteine oxygenase identified by the method of claim
 27. 33. A method of modulating angiogenesis in a subject, comprising administration of a modulator identified by the method of claim
 27. 34. A method of treating an angiogenesis-related disorder in a subject comprising administering to a subject a modulator of angiogenesis, thereby treating the angiogenesis-related disorder in the subject.
 35. The method of claim 34, wherein the modulator inhibits cysteine oxygenase.
 36. The method of claim 34, wherein the subject is a mammal.
 37. The method of claim 34, wherein the modulator of angiogenesis inhibits angiogenesis in the subject.
 38. The method of claim 34, wherein the angiogenesis-related disorder is cancer, tumors, rheumatoid arthritis, psoriasis or metastasis of cancerous cells in the subject.
 39. The method of claim 34, wherein the modulator of angiogenesis stimulates angiogenesis in the subject.
 40. The method of claim 34, wherein the angiogenesis-related disorder is coronary artery disease, stroke or delayed wound healing. 